First, the following description will discuss the background of the present invention concerning control for image quality in the image forming process.
In copying machines, etc. of the electrophotographic system, it is desirable to stabilize the density of a toner image that is formed on a copy sheet in order to obtain a copied image of high quality. However, in the electrophotographic process, characteristics of the exposure lamp, the photoreceptor, the toner, etc. are prone to change due to various factors, such as temperature, moisture, stains, wear, and property changes in materials; therefore, the density of toner image changes due to those changes. In order to solve this problem, there have been devised and put into practice various methods for compensating for the changed amount in the density of toner image by controlling the amount of exposure, the amount of charge of the photoreceptor, etc., during the processes from the exposure of an original document to the formation of the toner image.
For example, conventionally, a method for controlling a charging voltage based on a reference value that is obtained by optically detecting the density of toner image has been employed. In such a method, in the stage where the copy image quality in the copying machine is initially adjusted, the density of a toner image which is formed by a specific charge voltage value is detected by a reflective optical sensor, and the detected value is stored as a reference value for control. Thereafter, the density of toner image is detected in the same manner, if necessary, and the difference between the detected value and the reference value is converted into a compensating amount for charge voltage value of the photoreceptor. In this compensation, the following compensating characteristic is employed.
An image-forming operation in a common copying machine is classified into three processes, that is, the exposing process of the original document, the charging and exposing processes of the photoreceptor, and the developing process. FIG. 43 shows characteristics of those three processes.
FIG. 43(a) shows the relationship between the optical density (horizontal axis) of the original and the common logarithm of the amount of reflected light from the original (vertical axis). Suppose that the optical density is D, the amount of reflected light from the original is X, and the amount of light reflected from the background (white paper) of the original is Xm, the optical density is given by equation (1). EQU D=-log(X/Xm) (1)
FIG. 43(b) shows the relationship between the electric potential (horizontal axis) of the photoreceptor after exposure and the common logarithm of the amount of incident light to the photoreceptor (vertical axis), that is, the amount of reflected light from the original. Further, FIG. 43(c) shows the relationship between the electric potential of the photoreceptor (horizontal axis) and the density of toner image (vertical axis). Moreover, FIG. 43(d) shows the relationship between the optical density of original (horizontal axis) and the density of toner image (vertical axis). In FIG. 43, (d) is obtained by applying respective values in the order of (a).fwdarw.(b).fwdarw.(c), and the decision on the quality of copied picture is made by the use of this drawing.
Further, Japanese Examined Patent Publication No. 29502/1986 (Tokukoushou 61-29502) discloses a technique for controlling the exposing condition, developing condition and charging condition in response to signals corresponding to bright and dark portions of an image formed on the photoreceptor. With this arrangement, a signal corresponding to the bright portion of the image is detected, and the exposing condition for forming a latent image or the developing condition for making the latent image visible is controlled in response to the signal. Further, a signal corresponding to the dark portion of the image is detected, and the charging condition on the electrostatic latent image forming member is controlled in response to the signal.
Moreover, there has been proposed another method wherein the amount of Change in temperature and humidity and the sensitivity change in the photoreceptor in the copying machine, which form some of factors that would cause changes in the characteristics, are measured, and from the results, a compensating value for the control voltage is found (see "Electrophotographic Process Control Method by the use of Neural Network and Theory of Fuzzy", Image Electronics Society, A Technical Report 91-05-05).
The background as described above is not only related to copying machines, but also related to printers and facsimiles wherein the electrophotographic system is adopted. In copying machines, the photoreceptor is exposed by reflected light from an original; whereas in printers and facsimiles, exposure is conducted by changing the output of the exposing device using a laser light beam, etc. in response to an image signal inputted thereto.
However, in the above-mentioned method wherein the change in image quality is compensated for by measuring the change in the density of toner images, it is difficult to specify which portion of the image forming system, that is, exposure, charging, and developing, is responsible for the relevant change in image quality. For this reason, this method fails to compensate for a plurality of control values in a unified manner, thereby making it difficult to always determine best-suited control values. The reasons are described as follows:
Generally, even if the apply voltage for the exposure lamp and the output voltage of high-voltage power supply that is connected to the charging device are maintained constant, the respective characteristics, shown in FIGS. 43(a) through 43(c), change due to changes in temperature and humidity, wear of the photoreceptor, stains on the exposure lamp and the charging device, or other reasons.
For example, when only the amount of exposure changes to shift the characteristic of (a) from curve (1) indicated by a solid line to curve (2) indicated by a broken line, the characteristic of (d) changes from curve (3) indicated by a solid line to curve (4) indicated by a broken line. The characteristic of curve (3) provides good image quality in copied images; however, the characteristic of curve (4) causes the density to become higher as a whole, thereby making copied images darker.
Moreover, when only the electric potential of photoreceptor changes to shift the characteristic of (b) from curve (5) indicated by a solid line to curve (6) indicated by a broken line, the characteristic of (d) changes from curve (3) to curve (7) indicated by an alternate long and short dashes line. This causes the density to become lower as a whole, thereby making copied images lighter.
As described above, it is impossible to maintain the image quality of copied images in a good state for a long time merely by keeping control voltage values associated with exposure, charging, etc. at constant values.
Even in a technique disclosed in Japanese Examined Patent Publication No. 29502/1986 (Tokukoushou 61-29502), the respective conditions of exposure, charging and developing are individually controlled by signals derived from two types of images. Therefore, for example, in the case where the charging condition is changed, even if the characteristic associated with charging becomes better, the characteristic associated with exposure becomes worse conversely. As will be also described below, this fails to provide a sufficient compensation for the exposure condition, resulting in failure in providing optimum image quality.
With regard to the characteristic of (d), the bright portion, which is explained in the above patent publication as such, corresponds to an image that is derived from portion A having document densities of not more than 0.5, and the dark portion corresponds to an image that is derived from portion B having document densities of not less than 1.0. In (d), when only the exposure characteristic changes, the change in the bright portion (portion A) becomes greater as indicated by curve (4), and when only the charging characteristic changes, the change in the dark portion (portion B) becomes greater as indicated by curve (7). Therefore, in such a case, since the relationship between the change in each condition and the change in toner density is comparatively clear, the density compensation which is provided by controlling each condition individually is effectively adopted.
However, in the case where the exposure characteristic in (a) changes from curve (1) to curve (2) while the charging characteristic in (b) changes from curve (5) to curve (6), these changes make curve (8) as indicated by an alternate long and two short dashes line in (d). In other words, at portion A, since the charging characteristic and the exposure characteristic are cancelled, the change in density characteristic becomes small. At portion B, the effect of the change in the charging characteristic is so great that the effect of the change in the exposure characteristic hardly appears.
The density characteristic between document and toner, which is measured by the above compensating method, is indicated by "x" in FIG. 11 based on simulator experience, and these are connected to form a curve indicated by an alternate long and short dashes line. In FIG. 11, the horizontal axis represents the document density and the vertical axis represents the toner density. FIG. 11 shows that the characteristic obtained by the above simulator experience has a large deviation from the initial characteristic that is indicated by a solid line; this shows that the compensation is not carried out accurately.
Furthermore, in another method wherein ambient changes in temperature, humidity, etc. are measured, it is necessary to install a temperature sensor, a humidity sensor, an electric potential sensor, etc. Therefore, this method increases cost of electrophotographic apparatuses, and needs extra space to accommodate those sensors in the apparatus, thereby arising adverse effects on the compactness and maintenance of electrophotographic apparatuses. In addition to this problem, since it is difficult for the sensors to measure stains on the exposure lamps, etc., the above method fails to deal with indefinite factors such as stains.
The following description will discuss the background of the present invention concerning the condition setting on the image formation.
In a copying machine, normal copying is carried out in a standard mode wherein copying conditions such as quality in copied images are set at standard values. However, when copying is carried out in the standard mode, the resulting copies do not necessarily have a satisfactory image quality because the optimum setting on copying conditions differs depending on images of original documents and tastes of individual users. Taking account of the above problems, some conventional copying machines, such as described below, are designed so that the setting on the copying conditions can be changed.
For example, in copying machines having a function, so-called job memory, copying conditions are preliminarily registered through the function, and upon copying, the registered copying conditions are accessed on demand.
Moreover, Japanese Laid-Open Patent Publication No. 15866/1991 (Tokukaihei 3-15866) discloses a copying machine which informs the necessity of using the automatic document feeder in accordance with its copying history. More specifically, this copying machine stores its copying history such as document sizes and the number of copy sheets, every time copying is operated. Then, the frequency of document exchanges that have been successively executed within a predetermined period is found in accordance with the copying history, and a judgement is made as to the necessity of the automatic document feeder based on the frequency.
Furthermore, Japanese Laid-Open Patent Publication No. 273650/1992 (Tokukaihei 4-273650) discloses a copying machine which compares image data of a copied image of an original document with image data of the original document and adjusts the quality of copied images based on the results of the comparison.
However, in the copying machines having the job memory function, an operation on the control panel is necessary every time a copying condition is registered or accessed; this has made the operation complicated. Also, in copying machines that are disclosed in Japanese Laid-Open Patent Publications 15866/1991 and 273650/1992 (Tokukaihei 3-15866 and 4-273650), the number of parameters is decreased so that functions used for the setting on copying conditions can be simplified. In this arrangement, the relationship between the image information on an original document and the user's favorite color tone, density, etc. on the original document can not be inputted as copying conditions because too many parameters are required.
In order to solve this problem, a prospective method is to store image information on an original document and a copying condition on a one-to-one basis with each other, and to automatically set copying conditions based on the image information read from the original document. However, this method requires a memory with a large capacity to cover the increase in the number of stored data. Further, in the case where image information that has been read is different from the stored image information, a compensation by the use of approximation is carried out; this requires numerous stored data in order to make the accurate approximation. Moreover, if a memory with a large capacity is provided, it is difficult to access the data instantaneously without devising an appropriate data construction for the storage.